It is well known that it requires 538 calories to convert one gram of water at 212.degree. F. to steam at the same temperature. Once the heat is absorbed in the conversion process from liquid to gas, it is referred to as latent heat of vaporization. Upon condensing, an equivalent amount of heat is given off and, not suprisingly, it is referred to as heat of condensation.
It is common knowledge in the art that residential exhausting flue gases at relatively high temperatures such as, for example, 500.degree. F., represent a substantial amount of heat loss to the outside. By extracting heat from the flue gases before exhausting them, not only is sensible heat recovered but the lowering in temperature also results in condensation of the products of combustion and recovery of some of the latent heat of vaporization. Furnaces which recover sensible heat and a portion of the latent heat of vaporization from the flue gases before exhausting them to the outside have been referred to as recuperative or condensing furnaces.
Because of the substantial losses through the stack of a non-condensing furnace, the maximum efficiency for these systems is generally considered to be in the range from 75-85%. On a seasonal basis, this efficiency range is reduced because useable heat is also lost up the chimney through a draft hood during the cool-down period at the end of each heating cycle. Condensing or recuperative furnaces, on the other hand, are not substantially limited by these two factors and accordingly, efficiencies substantially above 90% are attainable. With the rapid rise in cost of fuel in recent years, the efficiency of a furnace has been much more important.
U.S. Pat. No. 4,178,907 shows a forced air heating system wherein the flue gases are channelled through a heat exchanger to remove heat therefrom before exhausting to the outside. In this system, the recirculating air from the intake duct passes first across a heat exchanger operating from the heat of combustion and then past the heat exchanger receiving the flue gases. A disadvantage of this system is that when the recirculating air comes in contact with the flue gas heat exchanger, the air has already been heated by the other heat exchanger. Accordingly, the temperature differential between the recirculating air and the flue gases is reduced thereby substantially decreasing the thermal transfer from the flue gases. Stated differently, recovery of sensible heat and latent heat of vaporization from the flue gases will not be at the same high rate as if the recirculating air were at a lower temperature.
U.S. Pat. No. 4,122,999 also shows a forced air heating system wherein some of the heat is extracted from the flue gases before they are exhausted to the outside. The flue gases first pass through a heat exchanger which is positioned within the hot air duct and then through a second heat exchanger positioned in the return air duct to the recirculation blower. This system, however, does not provide optimum fuel efficiency.
U.S. Pat. No. 4,261,326 discloses a three cell furnace wherein two of the cells have a burner and the third cell operates as a heat exchanger to extract heat from the flue gases before exhausting from the system. With this system, however, as described earlier, the recirculation air does not pass over the recuperative heat exchanger first; rather, it passes across all three cells in parallel. Accordingly, the extraction of heat from the recuperative cell is not at optimum efficiency.